1. Technical Field
The present invention relates to a power amplifier, and more particularly, to a power amplifier capable of restraining power and a method for restraining the power of the power amplifier.
2. Description of Related Art
Conventional power amplifiers may be classified into Class D power amplifiers and Class AB power amplifiers, among which the Class D power amplifiers are configured to output pulse modulated signals and the Class AB power amplifiers are configured to output linear signals. Owing to the advantage of high efficiency thereof, the Class D power amplifiers have now become a byword for power amplifiers.
Referring to FIG. 1, there is shown a circuit diagram of a conventional Class D power amplifier in which a dual-terminal output configuration is formed by a positive output terminal and a negative output terminal. This Class D power amplifier is advantageous in that by using the dual-terminal output configuration to drive a speaker 307, the driving power can be increased so that the speaker 307 can play the audio sound more efficiently.
Operation of this Class D power amplifier in the dual-terminal output configuration will now be described with reference to a single terminal as follows. From the terminal Vin, audio signals are inputted into an operational amplifier 301 and, by means of an integrator composed of the operational amplifier 301, a resistor R16 and a capacitor Cinl, the audio signals are transformed into self-oscillating triangular wave signals. Furthermore, square wave signals Vsquarel are inputted into the operational amplifier 301 via the terminal Rinl to synchronize the frequency of the triangular wave signals. Then, pulse-modulated signals are outputted through a comparator 302 to a pre-drive 305 which, according to the pulse-modulated signals, controls the on/off status of a power CMOS circuit 306 disposed at the output terminal, thereby to output signals for driving the speaker 307.
What described above is the circuit construction and operation of the conventional Class D power amplifier. The conventional Class D power amplifier is advantageous in that, the square wave signals are used as an input to control a carrier frequency of the output signals, and design of a square wave circuit is much simpler than that of a triangular wave circuit that is conventionally used. This saves room for circuits in the IC design and consequently reduces cost of the circuit layout. However, although the dual-terminal output configuration of the Class D power amplifier in which both the positive output terminal and the negative output terminal output signals can provide increased power for driving a load (e.g., the speaker), there is a risk that the load might be damaged due to an excessively large output power if a device for restraining power amplification is not provided.
Accordingly, in view of the shortcomings of the conventional Class D power amplifiers, the inventor of the present invention has made great efforts to make improvement thereon and finally provides an improved power amplifier and a method for restraining power of the improved power amplifier.